Sound enhancing system for musical instruments

ABSTRACT

Apparatus for enhancing the sound quality of musical instruments and of electronic sound sources is disclosed. Electrical signals representative of sound are modulated in a predetermined manner to increase their spectral content and are then processed by a resonance network which alters the amplitude and phase relationships of the various harmonic signal components. The processed signals are then demodulated in accordance with the modulation method used, thereby recreating a distorted replica of the original tone which is rich in harmonics that are uncorrelated in amplitude and in phase with each other. Upon conversion to sound this demodulated signal results in a pleasing tone.

Patent 1 1 SOUND ENHANCING SYSTEM FOR MUSICAL INSTRUMENTS [4 June 25,1974 3,688,010 8/1972 Freeman 84/111 Primary ExaminerKathleen H. Claffy[75] Inventor: g x g f Ngw Assistant Examiner.lon Bradford Leaheey rowence Attorney, Agent, or Firm-G. E. Murphy [73] Assignee: Bell TelephoneLaboratories,

Incorporated, Murray Hill, NJ. [57] ABSTRACT [22] Filed: No 24, 1972Apparatus for enhancing the sound quality of musical instruments and ofelectronic sound sources is dis- PP -Z 309,203 closed. Electricalsignals representative of sound are modulated in a predetermined mannerto increase [52 us. (:1. 179/1 M, 179/1 D 84/1.04 their Spectral Contentand are Processed by 9 [51] Int. Cl. G10h 5/00 onance network whichalters the amplitude and Phase [58 Field of Search 179/1 M 1 D 1 SA'relationships of the various harmonic Signal compo- 84/H6 L04 1 nentsvThe processed signals are then demodulated in accordance with themodulation method used, thereby [56] References Cited recreating adistorted replica of the original tone which is rich in harmonics thatare uncorrelated in UNITED STATES PATENTS amplitude and in phase witheach other. Upon converg i 67 sion to sound this demodulated signalresults in a 1'00 5.... 3,667,047 5/1972 lwasaki 179/1 SA Pleasmg tone3,668,294 6/1972 Kameoka 179/1 SA 15 Claims, 8 Drawing Figures 20RESONANCE l0 NETWORK r 3 SIGNAL AUDIO SOURCE CONVERTOR PATENTEDJUHZS1914 3.8193361 sum 1 or 3 FIG.

'RESONANCE' NETWORK SIGNAL AUDIO SOURCE T CONVERTOR u. 0 5 Z 5 2000- O EA IOOO P .IX

1 1 I I I 1 I I o 4 8 l2 I6 20 x= PEAK NUMBER so 70 7 so I I IPATENTEDJUHZS I974 3L81 9.861

SHEET 2 0F 3 FIG. 4

L20 RESONANCE NETWORK f F/G. .5

BASIC fi BASIC I NETWORK I WY I, J 72 ::13 ;1 m 1 \BASIC NETWORK y {l5{l6 7 {l7 (l8 rlg SIGNAL RESONANCE AUDIO SOURCE EXPANDER NETWORKCMPRESSR CONVERTOR PAIENTEUJUII 25 I874 SHEET 3 IIF 3 SIGNAL FREQUENCYSIGNAL FREQUENCY FM MODULATION SIGNAL *c FREQUENCY MODIFIED FM MODULATEDSIGNAL c FREQUENCY DEMODULATED SIGNAL MEDE ESE FREQUENCY SOUND ENHANCINGSYSTEM FOR MUSICAL INSTRUMENTS BACKGROUND OF THE INVENTION Thisinvention relates to signal processing and, in particular, to processingsystems for modifying the sound quality of musical instruments.

Electronic means are used extensively to enhance the quality andrichness of sound that is generated by musical instruments. This is doneby obtaining an electrical signal representation of the sound to beproduced, modifying it according to some method, and converting themodified signal to sound.

Filters are commonly used as signal modifiers to achieve soundenhancement, as for example, bass boost, treble boost, loudness control,etc. More sophisticated approaches use nonlinear devices as the signalmodifiers to increase the harmonic content of the basic signal. Forexample, U.S. Pat. No. 3,213,180, issued to .I. C. Cookerly on Oct. 19,1965 discloses one system for generating signals, responsive to musicalinstruments, having a qiven waveform and frequency response. In thissystem, as in other systems where enrichment of sound is achievedthrough nonlinearities, care must be taken to choose the proper approachbecause some nonlinearities (i.e., distortions) produce a less pleasingsound rather than a more pleasing sound. One reason for this is thatdiscordant sound includes harmonics that are phase correlated'to theirfundamentals, while pleasing sound generally has uncorrelated harmonics.This characteristic of sounds is described in an article entitledRegarding the Sound Quality of Violins and a Scientific Basis for ViolinConstruction," Journal of the Acoustical Society of America Vol. 29,July 1957, page 817, by H. Meinel, where it is pointed out that a violinexhibits a plurality of resonances that are not harmonically related toeach other and that the position and height of these resonances areimportant parameters with respect to the instruments tone quality.

Therefore, a need exists to rigorously classify and characterize theseparameters, and to develop apparatus which takes advantage of the soundimprovements made possible by the manipulation of said parameters.

SUMMARY OF THE INVENTION It is therefore an object of this invention toenrich and to enhance the quality of sound emitted by musicalinstruments by providing apparatus for affecting the pertinentparameters of sound.

It is another object of this invention to provide a network thatexhibits resonances at various points in the frequency spectrum at suchlocations and of such magnitude as to enhance the sound quality ofmusical instruments that are used in conjunction with the invention.

Therefore, in accordance with this invention, enrichment and enhancementof sound are achieved through alterations of the pertinent parameters ofsound. One embodiment of this invention comprises a signal source, aspectrum expander coupled to the signal source, a resonance networkcoupled to the spectrum expander, a spectrum compressor coupled to theresonance network, and an audio convertor coupled to the spectrumcompressor.

It has been discovered that a musical instrument possesses a pleasantsound if it exhibits a plurality of peaks in its frequency response.Accordingly, the resonance network of this invention exhibits peaks andvalleys in its frequency response where the peaks have the followingproperty;

a. The frequencies of peak response occurrences must be spaced in anonuniform manner with respect to the harmonic frequencies of any tone.The nonuniform peak spacing must insure that each harmonic of a giventone is on an up slope, down slope, peak, or valley of the frequencyresponse curve, without any strong correlation in amplitude or phase ofeach harmonic.

b. Theattenuated response away from the peak must be sufficiently largeso that the response curve is steep almost everywhere; that is to say,the magnitude of the derivative of the response curve must be largealmost everywhere.

c. The peaks must be approximately at one uniform level and theintervening valleys must be approximately at another unifonn level suchthat the ratio of the peak level to the valley level is between 10 db to15 db.

The spectum compressor of this invention is such that if connected tothe spectrum expander, the output signal developed in a recreation ofthe spectrum expanders input signal. For example the spectrum expandermay be an AM modulator of carrier frequency F and the spectrumcompressor may be an AM demodulator of the same carrier frequency.

Operation of the system proceeds in the following manner. A signalemanating from the signal source is modified in a predetermined mannerby the spectrum expander, thereby adding spectral lines to the originalspectrum. The signal thus modified then passes through the resonancenetwork, wherein the signals various frequency components are changed inamplitude, and in phase, in an uncorrelated fashion from each other(because of the nonuniform spacings of the resonance networks peaks).The spectrum compressor subsequently modifies the signal from theresonance network and recreates a distorted replica of the original tonethat contains harmonics and subharmonics of the tones fundamentalfrequency. This signal is then transformed to sound by the audioconvertor, such as a loud speaker resulting in a pleasing sound.

One feature of this invention is the complete latitude available in theselection of the spectrum expander and of the spectrum compressor. Infact, both may be omitted from the sound enhancing system, particularlywhen the original tones to be enhanced contain more than a singlefrequency.

The operation and features of the present invention will become moreapparent upon perusal of the following description taken in conjunctionwith the accompanying drawing wherein:

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 illustrates a block diagram of asimple sound enhancing system in accordance with the invention;

FIG. 2 illustrates a graph of peak frequencies versus peak number whichis used as an aid for designing the resonance network in FIG. 1;

FIG. 3 illustrates the frequency response of a number of basic networks,and the composite frequency response of the resonance network of FIG. 1;

FIG. 4 illustrates an embodiment of the resonance network used in theapparatus of FIG. 1;

FIG. 5 illustrates a basic network which can be used within saidresonance network;

FIG. 6 is a block diagram depicting an alternative sound enhancingsystem;

FIG. 7 illustrates a possible set of spectra appearing at various pointswithin the system shown in FIG. 6; and

FIG. 8 illustrates an alternate embodiment of the resonance network usedin the apparatus of FIG. 1.

DETAILED DESCRIPTION FIG. 1 depicts apparatus for enhancing the tonequality of musical instruments that produce sounds rich in harmonics,comprising signal source 10, resonance network 20 responsive to theoutput of the signal source, and audio convertor 30 responsive to theresonance network output.

Signals emanating from a musical instrument, as for example, from abowed or plucked string instrument, are converted to electrical signalsin signal source 10. Any of a variety of known transducers may be usedfor this purpose, e.g., magnetic or electrostatic pick-up devices. Thesesignals are then modified in resonance network 20 by altering theamplitude and phase characteristics of the signal as a function offrequency, in a manner to be fully described herein, and in accordancewith the desired peak and valley response as described above. Followingthe signal modification the electrical signal is converted in audioconvertor 30, resulting in a pleasant sound.

To achieve the desired peak and valley frequency characteristic of thesignal, resonance network 20 transfer function must exhibit apreselected frequency response. That is, the resonance network mustexhibit response peaks at nonuniform frequency spacing, the responsecurve must be steep" almost everywhere, and the peak to valley ratio ofthe response curve should be between 10 dB and dB as discussed above.

Specification of resonance network parameters can proceed via a numberof ways, one of which is described herein by way of a specific example.

First. the nonuniform frequency spacing between response peaks of theresonance network is obtained by defining an equation of peak frequency,f,,, versus peak number, x, which describes the general distribution ofpeaks that may be required to achieve a particular number anddistribution of peaks within the audio range. The peak frequency, f,,,can be a nonstraight line' equation, having, for example, the generalform f,,(.r) a r a,.r a x (1) containing any number of terms, wherein ais not the only nonzero coefficient. For example, to enhance the soundof a violin string, it is beneficial to design peaks within the audiorange. For that purpose, f., may be the exponential function fp=125 mu:(2)

which may also be described as f,,= l( l 0.174 x (0. l74x) /2!(0.l74x)"/n! (a) This function s nonlinear character is well illustratedby the plot of FIG. 2, wherefrom the frequencies of peak occurrence canbe determined by raising a perpendicular, line 40 of FIG. 2, from achosen peak number, to

intersect with the plotted curve, and by extending a horizontal line,such as line 50, of FIG. 2, toward the vertical axis, to detennine thepeaks frequency. Alternatively, equation 2, characterizing f,,(x), canbe directly solved for any value of x, as for example, for x 8, f E 500.1

One configuration of resonance network 20 suitable for this invention isshown, in FIG. 4, where the input to the resonance network drives aplurality of basic networks such as networks 21, 22, 23, and 24, andwhere the outputs of said basic networks are summed in conventionalanalog summer 14. A suitable summer is described by G. E. Tobey et al inOperational Amplifiers Design and Description, McGraw-I-Iill BookCompany, 197], page 429. Another configuration of resonance network 20may contain basic networks interconnected in cascade.

Once the peak frequencies of resonance network 20 are known, the networkcan be constructed to yield the desired peaks and the desired peak tovalley ratio by properly proportioning a set of basic networks andcombining them in a predetermined manner as shown, for example, in FIG.4, wherein each basic network is selected to contribute one peak in theresonance networks composite response and a controllably attenuatedresponse at frequencies other than the peak frequency. These peaks areselected to correspond to the frequencies determined from the graph ofFIG. 2. FIG. 3 shows response curves 60, 70, and 80, respectively of thebasic networks that generate the lOth, 19th and 20th peak, respectively,of the composite response curve 90.

The use of basic networks to configure resonance network 20 imposes therequirement that each basic network must possess one dominant resonance,that the resonance frequency of the basic network be controlled, andthat the Q factor of the basic network also be controlled, so thatresonance network 20 exhibits a composite frequency response, as shownby curve in FIG. 3, with a 10 to 15 db peak-to-valley ratio.

With resonance network 20 constructed as shown in FIG. 4, the basicnetwork must exhibit maximum signal transfer at resonance. Accordingly,a conventional second order filter, as illustrated in FIG. 5, can serveas the basic network. From equation 2 the required resonance frequencyof each second order filter can be determined. The Q of each secondorder filter is set to obtain a desired peak height for that filter inorder to achieve a peak valley ratio of 10 to 15 db. This determinationis shown in an appendix to this specification.

An alternate method may be used to obtain the resonance networkspecification by the use of basic networks, connected in cascade, asshown in FIG. 8. In such an embodiment each basic network must exhibit aminimum signal transfer at the valleys of the resonance networkscomposite frequency respone. An equation similar to equation 2 can beused to specify the required valley frequencies. Further, it would beobvious to those skilled in the art that various combinations ofparallel and serial interconnections of a num ber of passive or active,basic networks can be specified to realize this invention. Also, itwould be equally obvious to those skilled in the art that equation 2 cantake on many other forms, such as f, (x) a,,x B where x is the peaknumber, a, is positive and non zero, and B is a random variableuniformly distributed between 1*:

creasing the spectrum content of the signal, and applies the modifiedsignal to resonance network 17. In this system, resonance network 17characteristic response is similar to the characteristic response ofresonance network 20 except that is is centered about the carrierfrequency of the spectrum expander. The output of resonance network 17is applied to spectrum compressor 18 which performs the inverse functionof spectrum expander l6, and the altered signal of spectrum compressorI8 is applied to audio convertor 19, where the signal is transformed tosound.

A more thorough understanding of the system in FIG. 6 and its operationmay be obtained by observing the spectra shapes at various points in thesystem. By way of example, FIG. 7 illustrates a possible set of spectracorresponding to the system of FIG. 6'where the spectrum expander is anFM modulator of carrier frequency f and the spectrum compressor is an FMdemodulator of the same carrier frequency.

Assume that signal source produces a single frequency tone having asingle line spectrum as shown in FIG. 7A. The spectrum of the signal atthe output of modulator 16 is expanded as shown in FIG. 7B in accordancewith the standard Bessel function expansion of FM modulated signals.This spectrum is then applied to resonance network 17 which alters thespectrum irregularly, as shown in FIG. 7C, in accordance with thetransfer characteristics of resonance network 17, shown by curve 200 ofFIG. 7C. Demodulator l8 operates on the spectrum shown in FIG. 7C andproduces a baseband signal, shown in FIG. 7D. This baseband signal hasthe same fundamental frequency as the original tone from signal source15, but also has additional spectral lines which do not have a strongcorrelation in amplitude or in phase to the signal of source IS. Thesignal that corresponds to the spectrum shown in FIG. 7D, when convertedto sound by audio convertor 19, produces an interesting and a pleasingtone.

APPENDIX A resonance network comprising second order filters as thebasic network exhibits a frequency response that is characterized by(0mm 5 l/ l Substituting for e )mln E i/[( r+1- 1) 3] and the peak tovalley ratio. P/V, is

The bandwidth of the filter to achieve a given peak to valley ratio isthus 1 t+l 'r)/ V P/V and the Q of the filter, w,/2B,, is

If peaks of the resonance network follow the equation This resultindicates that a resonance network with peaks spaced according toequation Ae and constructed with second order filters requires filterswith identical Qs.

Another interesting feature of equation A3 is that w /w, e, which is aconstant.

In the example given within the specification, for a peak to valleyratio of 10 dB, and f,,(x) e the requiredQ of each and every filter isWhat is claimed is:

1. Apparatus for enhancing the sound quality of applied signalscomprising:

a plurality of basic networks responsive to said applied signals. eachexhibiting a resonance at a preselected frequency and a controllablediminished response at frequencies other than said resonance frequency,and means for combining the output signals of said basic networks todevelop a composite frequency response characterized by spectral peaksat a substantially unifonn first level and alternating valleys at asubstantially uniform second level, wherein said spectral peaks occur atsubstan tially nonuniform frequency spacings and said valleys,interposed between said peaks, also occur at substantially nonuniformfrequency spacings. I

2. The apparatus defined in claim 1 wherein the ratio of said firstlevel to said second level is between IOdb and l5db.

3. The apparatus defined in claim 1 wherein each of said basic networksexhibits maximum signal transfer at its resonance frequency.

4. The apparatus defined in claim 1 wherein each of said basic networksis a second order filter and in which the ratio of peak frequency of anytwo adjacent filters is a constant e, and the quality factor of eachfilter is equal to 2' V P/V/(el where P/V represents the desiredpeak-to-valley ratio of said resonance network.

5. The apparatus defined in claim 1 wherein the peak frequencies,(f,,(x), of the composite frequency response of said basic networks aredetermined by the nonlinear equation f,,(x) a x-l-a,x +a x where a isnot the only non-zero coefficient, and x is the peak number.

6. The apparatus defined in claim 1 wherein the peak frequencies.f,,(x), of the composite frequency response of said resonance networksare determined by the equation [,(x) a x B, where a is positive nonzero,x is the peak number, and B is a random variable uniformly distributedbetween i d 7. Apparatus for enhancing the sound quality of appliedsignals comprising:

a resonance network including a plurality of basic networks responsiveto said applied signals and means for combining the output signals ofsaid basic networks, each of said basic networks exhibiting a resonanceat a preselected frequency and a controllable diminished response atfrequencies other than said resonance frequency, said resonance networkexhibiting a frequency response that contains spectral peaks, andvalleys situated therebetween, wherein said peaks, at a substantiallyuniform first level, occur at substantially nonuniform frequency spacingand wherein said valleys are at a substantially uniform second level.

8. The apparatus defined in claim 7, further comprising a spectrumexpander electrically interposed between said applied signals and saidresonance network and a spectrum compressor responsive to said resonancenetwork.

9. The apparatus defined in claim 8 wherein said spectrum expander andsaid spectrum compressor comprise a modulator and a demodulator,respectively, with the same carrier signal frequency.

10. Apparatus as defined in claim 9 wherein said modulator is an FMmodulator and wherein said demodulator is an FM demodulator.

11. Apparatus for enhancing the sound quality of applied signalscomprising:

a resonance network including a plurality of basic networks responsiveto said applied signals and means for combining the output signals ofsaid basic networks, each of said basic networks exhibiting a resonanceat a preselected frequency and a controllable diminished response atfrequencies other than said resonance frequency, said resonance networkexhibiting a frequency response that contains spectral peaks at asubstantially uniform first level and valleys at a substantially uniformsecond level situated between said peaks, wherein said peaks occur atsubstantially nonuniform frequency spacings; and

an audio convertor responsive to the output signals of said resonancenetwork for converting electrical signals to sound.

12. Apparatus for enhancing the sound quality of applied signalscomprising:

a resonance network responsive to said applied signals including aplurality of basic networks interconnected in cascade, where each ofsaid basic networks exhibits a minimum signal transfer at its resonancefrequency and a controllable diminished response at frequencies otherthan said resonance frequency, said resonance frequency corresponding toa valley of the resonance networks frequency response, wherein thevalleys of said resonance networks frequency response occur atsubstantially nonuniform frequency spacings and are substantially at afirst uniform level and wherein said controllable diminished response isadjusted to provide peaks between said valleys having substantially asecond uniform level.

13. The apparatus defined in claim 12, further comprising a spectrumexpander electrically interposed between said applied signals and saidresonance network and a spectrum compressor responsive to said resonancenetwork.

14. The apparatus defined in claim 13 wherein said spectrum expander andsaid spectrum compressor comprise a modulator and demodulator,respectively, with the same carrier signal frequency.

15. Apparatus as defined in claim 14 wherein said modulator is an FMmodulator and wherein said demodulator is an FM demodulator.

UNITED STATES UFFICE CERTIFICATE OF CORRECTION Patent No. 1 ,851 DatedJune 25, 197 4 It is certified that error appears inthe above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 23, "qiven" should read --gi en- Column 2, line .26.,"in" should read -is-.

Column 5, line 61, "s -w should read "s -w llne 6 6, e W -W /2 shouldread e (w m )/2,

Column 6, line 1 after equation insert --orline 19, "H(s) 2 AB /e +Bshould read --H(s)- E AB /(e +B Signed and sealed this 12th day ofNovember 1974.

(SEAL) Attest:

C. MARSHALL DANN McCOY M. GIBSON JR.

Commissioner of Patents Attesting Officer USCOMM-DC 60376-P69 U.SGOVERNMENT PRINTING OFFICE I909 O-3S6-J34.

FORM PO-105O (10-69) UNITED STATES 1 mm UFFICE CERTIFICATE OF CORRECTIONPatent No. ,819,861 D ated v June 25 197 1 It is certified that errorappears in the abovei.dentl[ied patent and that said Letters Patent arehereby corrected as shown' below:

Column 1, line 23, "qiven" should read "given".

Column 2, line .26, "in" should read --is--. Column 5, line 61, "52 -wshould read -S2 -w line 66, "e W -W /2" should read --e (w -w )/2 Column6, line 1 after equation insert --or--;.

--H(s)- e AB /(e +B i) Signed and sealed this 12th day of November 1974.

(SEAL) Attest:

C. MARSHALL DANN McCOY M. GIBSON. JR.

Commissioner of Patents Attesting Officer USCOMM-DC 60376-P69 FORMPO-1050 (10-69) h u.s. covzmmzm rmm'ms ornc: nu 0-366-334.

1. Apparatus for enhancing the sound quality of applied signalscomprising: a plurality of basic networks responsive to said appliedsignals, each exhibiting a resonance at a preselected frequency and acontrollable diminished response at frequencies other than saidresonance frequency, and means for combining the output signals of saidbasic networks to develop a composite frequency response characterizedby spectral peaks at a substantially uniform first level and alternatingvalleys at a substantially uniform second level, wherein said spectralpeaks occur at substantially nonuniform frequency spacings and saidvalleys, interposed between said peaks, also occur at substantiallynonuniform frequency spacings.
 2. The apparatus defined in claim 1wherein the ratio of said first level to said second level is between10db and 15db.
 3. The apparatus defined in claim 1 wherein each of saidbasic networks exhibits maximum signal transfer at its resonancefrequency.
 4. The apparatus defined in claim 1 wherein each of saidbasic networks is a second order filter and in which the ratio of peakfrequency of any two adjacent filters is a constant ec, and the qualityfactor of each filter is equal to
 2. Square Root P/V/(ec-1), where P/Vrepresents the desired peak-to-valley ratio of said resonance network.5. The apparatus defined in claim 1 wherein the peak frequencies,(fp(x), of the composite frequency response of said basic networks aredetermined by the nonlinear equation fp(x) a0x+a1x2+a2x3+. . . , wherea0 is not the only non-zero coefficient, and x is the peak number. 6.The apparatus defined in claim 1 wherein the peak frequencies, fp(x), ofthe composite frequency response of said resonance networks aredetermined by the equation fp(x) a0x +B, where a0 is positive non-zero,x is the peak number, and B is a random variable uniformly distributedbetween + or - a0.
 7. Apparatus for enhancing the sound quality ofapplied signals comprising: a resonance network including a plurality ofbasic networks responsive to said applied signals and means forcombining the output signals of said basic networks, each of said basicnetworks exhibiting a resonance at a preselected frequency and acontrollable diminished response at frequencies other than saidresonance frequency, said resonance network exhibiting a frequencyresponse that contains spectral peaks, and valleys situatedtherebetween, wherein said peaks, at a substantially uniform firstlevel, occur at substantially nonuniform frequency spacing and whereinsaid valleys are at a substantially uniform second level.
 8. Theapparatus defined in claim 7, further comprising a spectrum expanderelectrically interposed between said applied signals and said resonancenetwork and a spectrum compressor responsive to said resonance network.9. The apparatus defined in claim 8 wherein said spectrum expander andsaid spectrum compressor comprise a modulator and a demodulator,respectively, with the same carrier signal frequency.
 10. Apparatus asdefined in claim 9 wherein said modulator is an FM modulator and whereinsaid demodulator is an FM demodulator.
 11. Apparatus for enhancing thesound quality of applied signals comprising: a resonance networkincluding a plurality of basic networks responsive to said appliedsignals and means for combining the output signals of said basicnetworks, each of said basic networks exhibiting a resonance at apreselected frequency and a controllable diminished response atfrequencies other than said resonance frequency, said resonance networkexhibiting a frequency responsE that contains spectral peaks at asubstantially uniform first level and valleys at a substantially uniformsecond level situated between said peaks, wherein said peaks occur atsubstantially nonuniform frequency spacings; and an audio convertorresponsive to the output signals of said resonance network forconverting electrical signals to sound.
 12. Apparatus for enhancing thesound quality of applied signals comprising: a resonance networkresponsive to said applied signals including a plurality of basicnetworks interconnected in cascade, where each of said basic networksexhibits a minimum signal transfer at its resonance frequency and acontrollable diminished response at frequencies other than saidresonance frequency, said resonance frequency corresponding to a valleyof the resonance network''s frequency response, wherein the valleys ofsaid resonance network''s frequency response occur at substantiallynonuniform frequency spacings and are substantially at a first uniformlevel and wherein said controllable diminished response is adjusted toprovide peaks between said valleys having substantially a second uniformlevel.
 13. The apparatus defined in claim 12, further comprising aspectrum expander electrically interposed between said applied signalsand said resonance network and a spectrum compressor responsive to saidresonance network.
 14. The apparatus defined in claim 13 wherein saidspectrum expander and said spectrum compressor comprise a modulator anddemodulator, respectively, with the same carrier signal frequency. 15.Apparatus as defined in claim 14 wherein said modulator is an FMmodulator and wherein said demodulator is an FM demodulator.